Coinbase co-founders Brian Armstrong (l) and Fred Ehrsam, sporting we-got-money grins. If you’re a bitcoin doubter, you might want to turn away. The doors to venture funding in bitcoin startups are about to swing wide open. Andreessen Horowitz has led a $25 million Series B investment in San Francisco-based Coinbase, the companies are announcing today , in what may very well be the largest ever venture investment in a bitcoin-related company. Coinbase previously raised nearly $7 million. Existing investors Union Square Ventures and Ribbit Capital will contribute some cash as well. And Union Square’s Fred Wilson and Andreessen Horowitz’s Chris Dixon are taking seats on Coinbase’s board of directors. Coinbase currently employs only eight people, but will add significantly to that total. It may also start to buy paid advertising to promote itself and bitcoin. Bitcoin is often...